1. Field of the Invention
The invention relates to an optical transmitter comprising a combination of an electro-optical converter and an opto-electrical converter which are optically coupled, which combination is included in the negative feedback path of a control loop, the input of the electro-optical converter being coupled to the input of the negative feedback path and the output of the opto-electrical converter being coupled to the output of the negative feedback path. The control loop includes a control amplifier, the output of the negative feedback path being coupled to an input of the control amplifier and the output of the control amplifier being coupled to a first input of combining means whose output is coupled to the input of the negative feedback path. Modulation signal is applied to a second input of the combining means.
2. Description of the Related Art
A transmitter of this type is described in U.S. Pat. No. 4,504,976. Transmitters of this type may be used, for example, in optical recording systems and in optical telecommunication systems.
In these systems the electro-optical converter is often desired to be biassed, so that it generates a certain amount of light even when a modulation signal is absent. In optical recording systems this is desired because the light generated when a modulation signal is absent is used to read information from a record carrier. In digital telecommunication systems the bias is desired because it increases the maximum attainable modulation frequency of the electro-optical converter. This is caused by the fact that the switch-on delay of a fully switched-off electro-optical converter is much longer than the delay in the increasing amount of light generated by an electro-optical converter to a given value from a bias level of the converter.
The electro-optical converters used in systems of this type generally have a strong threshold characteristic. This is to say, the current flowing through the electro-optical converter must exceed a specific threshold value before the converter emits light. This threshold is strongly temperature-dependent and furthermore exhibits a large variation per specimen.
Because the threshold of the current flowing through the electro-optical converter exhibits a large variation, it is not readily feasible to bias the amount of light generated by the electro-optical converter by means of a fixed bias current of the converter.
In order to nevertheless obtain a well-determined bias of the generated light, the electro-optical converter is generally included in the negative feedback path of of a control loop together with an opto-electrical converter, a fraction of the light emitted by the electro-optical converter being applied to the opto-electrical converter. Since the opto-electrical converter is included in the negative feedback path, the output signal of the opto-electrical converter and thus also the amount of light generated by the electro-optical converter is maintained at a predetermined value by the control loop.
Generally, the amount of light emitted by the electro-optical converter can be amplitude modulated by coupling the modulation signal to the input of the gain control amplifier. However, this leads to a restriction of the maximum permissible frequency of the modulation signal, because the gain control amplifier preferably has a high gain factor and thus a limited bandwidth.
In the transmitter known from said U.S. Pat. No. 4,504,976, the light emitted by the electro-optical converter is amplitude modulated by coupling the modulation signal directly to the input of the negative feedback path so that the limiting effect of the gain control amplifier on the maximum permissible frequency of the modulation signal is eliminated. A problem is then that the control loop tries to maintain the output signal of the negative feedback path and thus also the amount of light emitted by the electro-optical converter at a constant level, and is thus capable of reducing or even fully cancelling the effect of the modulation signal on the emitted amount of light.
In this known transmitter it is possible to apply a high-frequency modulation signal to the input of the negative feedback path and to arrange the control loop in such a way that it is only active for low frequencies, so that the control loop now no longer affects the relation between the modulation signal and the emitted amount of light. A drawback of this is that the transmitter is then unsuitable for modulation signals which comprise a DC component.